In recent years time division duplexing (TDD) has often been used in digital wireless communications systems. TDD is primarily used for transferring data, voice, and control information between two units. These units which are shown in FIG. 1 may be designated as base stations 101 or mobile units 100. Each base station 101 is generally a stationary unit while a mobile unit 100 is usually portable, hand held, mounted in a vehicle, etc. There are usually more mobile units 100 as shown in FIG. 1 than base stations 101. Therefore, minimizing cost, weight, and power consumption, is more important in the mobile units 100 than in the base stations 101.
TDD systems are defined as systems where the available frequency bandwidth is divided into frequency channels and each frequency channel is divided into time slots. One example of a TDD system is the Japanese digital cordless telephone (J-DCT), (also called personal handy phone--PHP) specified by the Japanese MOPT document RCR-28. In this system, frequency is divided into approximately sixteen, 300 kHz, frequency channels and each frequency channel is divided into eight time slots which together define a frame.
FIG. 2 shows the frame structure of the J-DCT system. The first four time slots 104 are reserved for transmissions from the mobile units and the next four time slots 106 are reserved for transmissions from one base station. Usually each transmit slot is rigidly paired with a receive slot so that each mobile unit is assigned one time slot during which it transmits to the base station and one time slot in which it receives transmissions from the base station during each frame. For instance, returning to FIG. 1, mobile units 100 designated as M1, M2, M3 and M4 are each assigned one of the time slots 104 in which the assigned mobile unit transmits to one of the base stations 101 such as the base station identified as BS1. Therefore, if the slots are numbered 1 to 8 in time as shown in FIG. 2, a mobile unit will transmit on a time slot from 1 to 4 and the base station will transmit back during a time slot exactly four slot times later. Accordingly, if a mobile unit transmits in time slot 2, the base station will transmit back to that mobile unit in time slot 6.
In TDD systems, each mobile unit typical transmits and receives on the same frequency. However, it is not unusual for a mobile unit in a TDD system to use transmit and receive frequencies which differ only by some fixed offset. By using a fixed offset frequency between the mobile unit's transmit and receive frequencies, the mobile unit's frequency synthesizer does not have to change frequencies between transmission and reception. It should be understood that the frequency offset can be applied to the receive signal prior to downconversion or to the transmit signal after upconversion to avoid modifying the synthesizer frequency, which may be both complicated and time-consuming.
TDMA (time division multiple access) differs from TDD in that the transmit and receive frequencies are often paired but different. U.S. digital cellular (IS-19B) is a TDMA system with six time slots per frequency channel. However, the receive and transmit frequency used in TDMA are often selected so that the offset technique described above can be used.
A fundamental advantage of using the same frequency during transmission and reception is that many channel distortions will be substantially symmetric. So equipment in the base station can be used to detect and compensate for channel distortions in its receiver and then pre-compensate for the distortions prior to transmission.
Fading caused by multipath is one factor which results in channel distortion. Another common factor resulting in channel distortion in mobile systems is frequency selective fading (also called time dispersion). This phenomenon results when there are two or more signal paths between the transmitter and receiver of comparable signal levels when at least one of the paths is significantly longer the another. If the maximum path difference is long enough the signal propagation delays may cause information to be missed or lost. Specifically, the receiver error rate is increased with signal propagation delay and that error cannot be reduced.
One way to reduce the effects of channel distortion is to use some form of antenna diversity. The base stations in the J-DCT system, for example, typically include spatial diversity by using two receive antennas. An example of the use of spatial diversity in such a system is disclosed in application Ser. No. 08/129,562 filed on Sep. 30, 1993. In general, antenna diversity systems recover data from the signal received on the better of the two antennas on a slot by slot basis. The base station transmits on the same antenna it last received on. This allows the mobile unit to be implemented without its own antenna diversity.
The advantages of TDD begin to be lost in more mobile systems. One problem is that if a mobile unit moves a significant percentage of a wavelength (in the J-DCT system a wavelength is approximately 15.8 cm) between its transmit slot time (when the base station receives) and its receive time slot (when the base station transmits) then the symmetry mentioned above no longer applies. Therefore, the greater number of time slots between a mobile unit's transmit and receive slots, the less the symmetry channel has with respect to distortion due to fading.
Typically reliable communications with diversity at the base station only, require that the maximum path difference between the transmit and receive paths change no more than 10%-20% of the wavelength at the operating frequency. In the J-DCT system, for instance, a slot time is approximately 625 .mu.sec. Therefore, the mobile unit cannot exceed a speed of about 14.2 miles per hour (10% of 15.8 cm is 1.58 cm, the mobile unit cannot travel more than 1.58 cm in 2.5 ms--the time between transmit and receive slots in the J-DCT system).
Another disadvantage of using TDD or TDMA in mobile communications systems is that the frame structures in such systems cannot be efficiently used for several reasons. In mobile systems such as the J-DCT system, a plurality of base stations are provided so that as a mobile unit moves away from one stationary base station towards a different stationary base station the communications link can be handed off from one base station to another. For instance, referring back to FIG. 1, if M3 is moving away from BS1 toward BS2, at some point it would become necessary for the communications link between BS1 and M3 to be handed off to BS2 so that M3 can still transmit and receive. There are many known techniques for performing this type of handoff. One technique is commonly referred to as a Mobile Assisted Handoff (MAHO).
Most base station handoff techniques assign one time slot in a frame to be used by the base station to transmit a base station ID on a control frequency. The base station transmits its ID in the assigned slot at some predetermined interval (e.g., every Nth frame where N is an integer). The mobile units monitor transmissions at the control frequency and make a determination related to the quality of any signal received at that frequency, e.g., signal strength indication. For example, if BS1 and M3 have established a communication link and it is determined that the signal received by M3 while monitoring the control frequency was transmitted from BS2 and that that signal's quality is better than the signal quality of signals received from BS1, then M3 would request a handoff from BS1 to BS2.
Monitoring other operating frequencies is also desirable in mobile communication systems so that each mobile unit/base station link can be optimized by transmitting and receiving on preferable operating frequencies. As described above in connection with the J-DCT system, 16 frequency channels are allocated with each frequency channel subdivided into four additional channels. Although, theoretically, each base station could communicate with 64 mobile units (16 frequency channels with four slot pairs each), typically the number is far less in actual operation. Thus, a number of channels may be open at any given time. For this reason, mobile units also monitor the other possible operating frequencies during time slots when the mobile unit is not transmitting to or receiving signals from the base station. If it is determined that a different operating frequency would provide better communications with the base station, the base station attempts to reassign the mobile unit's operating frequency using techniques which are generically referred to as Interference Avoidance Handoff. Such techniques are well known and widely used.
The slotted protocol described above has several performance disadvantages with respect to base station hand off and interference avoidance handoff. To accommodate such techniques in a TDD system the frequency synthesizer in each mobile unit would need to be capable of switching frequencies rather quickly to monitor the required frequencies. However, that capability would add a significant cost and undesirable complexity to the mobile units themselves. Therefore, most mobile units use a standard frequency synthesizer, such as those commercially available from Signetics, Motorola and other manufacturers.
For instance, in the J-DCT system, the frequency synthesizer used in the mobile units generally requires one time slot to switch frequencies. Referring back to FIG. 1, if M3 transmits to BS1 in time slot 2 and BS2 transmits to M3 in time slot 6, M3 would use time slots 1 and 3 to switch frequencies for transmission, and time slots 5 and 7 to switch frequencies for reception. Only time slots 3 and 8 would be available to monitor the control frequency and/or other frequency channels for a potential base station handoff or channel reassignment.
With only two non-contiguous time slots, it would be virtually impossible to efficiently monitor other operating frequencies to determine whether a mobile unit would be benefitted by a channel reassignment. Similarly, monitoring the control frequencies of other base stations to determine whether to handoff a mobile unit from one base station to another would also be ineffective where the monitoring time is broken up and short in duration. When the time bases associated with different base stations are not one base station may transmit its ID in the middle of another base station's time slot. In that case a mobile unit synchronized with one base station may not be able to receive and decode a different base station's transmission within the available time slots.
Therefore, there is a need to provide a mobile communications system which can effectively reduce channel degradation caused by fading and can efficiently monitor both other frequency channels and other base station transmissions so that known handoff techniques can be used effectively.